In the production of steel, various chemical elements are added to the molten ferrous metal to remove undesirable constituents, such as oxygen and sulphur, and to impart one or more desirable properties. These properties may include controlled grain size, improved mechanical strength, and corrosion resistance, among others. Such elements, called addition agents, may include various alloys of iron, termed ferroalloys. Two important addition agents are ferromanganese and ferrochromium which commonly used to remove and control sulphur and to introduce the elements manganese and chromium into molten steel. Typically ferromanganese has 78-84 percent manganese, a maximum of 7.5 percent carbon, and smaller percentages of other elements while ferrochromium has a similar proportion of chromium. Ferromanganese and ferrochromium are normally produced by refining or ferromanganese or ferrochromium ores having a manganese or chromium to iron ratio of about 7:1.
In one method of producing silicomanganese, standard grade, high carbon ferromanganese is produced from high-grade ore leaving up to 50% manganous oxide in the gangue-containing slag. After cooling and crushing, this slag is resmelted in a charge containing a lower grade of manganese, silica (which may be a constituent of the ore, reductant in the form of carbon, such as cool or coke, and possibly additional fluxes.
Various types of furnaces, such as electric arc furnaces, are employed in smelting processes. One prior art method of producing ferromanganese by reduction employed pairs of electric arc furnaces. The first furnace was charged with manganese-bearing ore along with other materials such as carbon which are required in the smelting process. Certain of the intermediate products obtained thereby are transported to a second furnace for further reduction which results in ferromanganese or silicomanganese along with other products.
Since the temperatures required for the reduction of ferromanganese and ferrochromium ores are relatively high, usually above 1400.degree. C., and since the heat transfer rate between bodies of disparate temperature is directly related to the temperature differences between the two bodies, it is advantageous from an energy conservation standpoint to retain any material being transported from a first stage to a second stage in a high temperature ambient. In smelting processes using separate furnaces, the material was cooled and crushed prior to delivery to the second furnace. As a result, considerable heat was lost, requiring the addition of this lost energy in the second furnace. Prior art smelting processes employing two furnaces also have substantial manpower requirements. Because of energy, equipment and manpower costs, prior art processes are not normally employed for smelting low-grade ferromanganese or ferrochromium ores which have a manganese or chromium to iron ratio of about 4.5:1.
Another disadvantage of prior art furnaces is that the carbon refractory brick used to line the vessel hearths was often absorbed into the product resulting in unpredictable variations in product chemistry as well as erosion of the refractory material itself.